One way to help make sure your top tier will be centered when you place it is to lightly press a different separator plate down into the icing in the center of the bottom tier, marking where your top tier will need to go.

When you place the top tier onto the bottom tier, make sure your fingers only touch the cake board/separator plate. Basically get the top tier centered over the bottom tier and as close to it as possible without your fingers touching either tier and then gently "drop" the top tier into place.

Once you've placed the top tier onto the bottom tier, do not move it around or adjust it.

When you pipe the bottom border around the top tier, it should cover up any accidental marks.